Most everyone from work will be at Julie's party. It's better to say almost in this sentence. Almost means nearly and most means the greater number—what we sometimes call a superlative (few, more, most). To make the sentence correct, we could say, "A few will go to Marvin's reception, but most of them will be at Julie's party."
I'm aware of the small things in grammar
because they add up to big things in writing.